Test system with reduced test contact interface resistance

ABSTRACT

An improved test system includes means for generating a contact wetting pulse and applying the contact wetting pulse to a network such that contact resistance at the interfaces between probes of the test system and terminals of the network is effectively lowered.

TECHNICAL FIELD

This invention relates generally to test systems and, more particularly,to a test system for verifying wiring in electronic products.

BACKGROUND OF THE INVENTION

In the manufacture of electronic products, it is generally required thatthe wiring of the product be verified by a test system. The electronicproducts which require such wire verification tests include substrates,printed wiring boards, multi-layer ceramic substrates, etc. Althoughthere are a multitude of wire verification tests that can be performed,the more common types of verification tests are the so-called continuityor short-circuit test and the so-called insulation or open-circuit test.Broadly, a continuity test verifies that an electrical connection orpath exists between terminals or points of an electronic product or adevice under test (DUT); and an insulation test verifies the absence ofelectrical connection or the existence of device leakage between pointsor sets of points of a DUT, i.e., an insulation test verifies thatpoints or sets of points are mutually isolated from each other. Theterminals or points are typically terminations of a network of the DUT.

Current day test systems commonly utilize solid state circuitry andemploy matrix-type switching systems capable of extremely high-speedoperation. For example, the switching system may be capable of beingconnected to over 260,000 terminals or points, thus allowing for thetest system to perform on the order of 6,000-8,000 tests per second.Moreover, the test systems typically have random access capabilitieswhich allow for accessing and testing of the various terminals or pointslocated on a DUT in any desired pattern and sequence.

The testing scheme generally utilized by these test systems to verifywiring includes applying electrical stimulus or signals between selectedpoints of a DUT, measuring the electrical resistance between the points,and comparing the measured resistance to a specified limit. Generally,the specified limit is a user defined maximum or minimum resistivequantity which takes into account known resistances of wires,connectors, etc., which is used to determine whether there should beconsidered a short-circuit or open-circuit existing between points of anelectronic product In this regard, a low limit is a resistive quantitythat the measured resistance must not exceed in order for there to beconsidered a short-circuit existing between the points; and a high limitis a resistive quantity that the measured resistance must exceed inorder for there to be considered an open-circuit existing between thepoints. As an example, the low limit may be on the order of 10 ohms, andthe high limit may be on the order of 10 Mohms.

A common problem associated with these test systems is the presence ofelectrical resistance at the contacts or interfaces between the probesof the test system and the terminals of the DUT being probed. Manycauses of this contact resistance are unavoidable. For instance, it isrequired that small diameter probes be utilized in order to contactsmall, closely spaced apart terminals; and it is further required thatrelatively poor electrically conducting probes be utilized in order tomaintain the required probe hardness to prevent breakage of the probeswhen a contact force is applied thereto. Inevitably, this contactresistance is included in the measurement of resistance between pointsof a DUT, thus causing inaccuracies when conducting short-circuit andopen-circuit tests. In other words, the resistance that is measuredbetween terminals of a DUT includes the resistance of the networkbetween the terminals, as well as the contact resistance at theinterfaces between the probes of the test system and the terminals ofthe DUT. Unfortunately, there is no practical method of distinguishingbetween these resistances when testing.

The inaccuracies caused by contact resistance may be the passing of anetwork that actually should fail, or the failing of a network thatactually should pass. One solution proposes compensating for contactresistance when setting the limits. In other words, adding acompensation resistance to the limit settings in order to compensate forcontact resistance. In this regard, the compensation resistance would bean approximation of the contact resistance. However, such a solution hasproven unsuccessful since contact resistance varies considerably fromone set of probes and terminals to another set of probes and terminals.A further problem is that contact resistance may vary from one test toanother even if the same set of probes is used to contact the same setof terminals. Measurements have shown that contact resistance can beanywhere between 1 ohm and 35 ohms. Moreover, measuring the contactresistance between each and every probe and terminal is highlyimpractical, since there can be between 5,000-35,000, or as much as over260,000, probe/terminal contacts that would require such a resistancemeasurement for a given electronic product.

Generally, contact resistance has been found to be a greater problem intesting for short-circuits or continuity as compared to testing foropen-circuits. Two examples of how contact resistance and compensationresistance can cause inaccurate results is given below:

    ______________________________________                                        low limit = 10 ohms                                                           compensation resistance = 5 ohms                                              compensated low limit = 15 ohms                                                       contact    network  measured                                                  resistance resistance                                                                             resistance                                        ______________________________________                                        Example I 9 ohms        8 ohms  17 ohms                                       Example II                                                                              2 ohms       12 ohms  14 ohms                                       ______________________________________                                    

Since the resistance of the network of Example I is 8 ohms, which isless than the low limit of 10 ohms, the network of Example I should passas a short-circuit. However, because of the contact resistance of 9ohms, the resistance measured by the test system is 17 ohms, which isgreater then the compensated low limit of 15 ohms. Therefore,erroneously, the network of Example I will fail the test, i.e., thenetwork will not be regarded as a short-circuit. Similarly, Example IIillustrates how compensating for contact resistance can allow a networkto pass as a short-circuit when it should actually fail.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a new and improvedtest system.

Another object of the invention is to provide an improved test systemwhich will perform open-circuit and short-circuit tests with increasedaccuracy.

Yet a further object of the present invention is to provide an improvedtest system which has the ability to lower contact resistance at theconnection between the test system and the device under test.

Still another object of the invention is to provide an improved testsystem which will perform open-circuit and short-circuit tests withincreased accuracy by lowering contact resistance at the interfacebetween the test system and the device under test.

In accordance with the present invention, there is provided an improvedtest system of the type which includes a plurality of switching cells.Each switching cell has a probe which electrically interfaces with aterminal of a device under test, and each switching cell is selectivelyand individually addressable for selecting a first and second switchingcell and corresponding first and second terminal. Input signals aregenerated by a voltage/current source and applied to a network undertest via the first switching cell and probe and via the first terminal.In turn, the network responsively generates output signals. The secondswitching cell and probe receive the output signals from the secondterminal and route the output signals to a comparator. The comparatorcompares the output signals with the input signals and calculatesdifferences in voltage and current, and the electrical resistance of thenetwork can thus be determined. According to the present invention,contact wetting means is further provided for generating a first pulseof increased magnitude, relative to the input signals. This first pulseis applied to the network via the switching cells and probes so as toeffectively lower contact resistance at the interfaces between theprobes and the terminals.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features, aspects and advantages will be morereadily apparent and better understood from the following detaileddescription of the invention, in which:

FIG. 1 is a block diagram of a prior art test system;

FIG. 2 is a schematic diagram of prior art switching cells that can beemployed in the test system of FIG. 1;

FIG. 3 is a block diagram of a test system in accordance with thepresent invention;

FIG. 4 is a schematic diagram of switching cells in accordance with thepresent invention; and

FIG. 5 is a flowchart illustrating the method of operation in accordancewith the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring initially to FIG. 1, there is shown a test system 10 inaccordance with the test system described hereinabove in the BACKGROUNDOF THE INVENTION. Those skilled in the testing arts will appreciate thatthe test system 10 is conventional and that the overall concepts andtesting scheme employed by the test system 10 are utilized by a numberof commercially available test systems.

The test system 10 includes a voltage/current source 15 which iselectrically connected to switching means 30. Switching means 30comprises a plurality of switching cells. In this regard, referringbriefly to FIG. 2, switching cells 45,50 are illustrative of the type ofcircuitry that can be utilized for the switching means 30, and it shouldbe understood that other types of switching circuitry can be used inlieu of the switching cells 45,50. Moreover, although only two switchingcells 45,50 are shown, there can be on the order of over 260,000switching cells included as part of the switching means 30, and suchswitching cells may be configured in the form of a matrix.

Each switching cell of the switching means 30 is electrically connectedto a terminal of the DUT 35, and each cell is in a normally open state.A controller 40 has the capability of electrically accessing oraddressing each individual switching cell for regulating closing of eachswitching cell so that testing can be conducted between any of theterminals of the DUT 35. More specifically, two switching cells may beaccessed or addressed for testing if the terminals corresponding tothese two cells has an electrical network existing therebetween thatmust be tested. Thus, the controller 40 includes logic circuitry asrequired for conducting tests, and may be manually operated orautomatically operated by computer.

Generally speaking, when a switching cell is closed, an electrical pathis provided so as to allow voltage and current input signals generatedby the voltage/current source 15 to be applied to the DUT 35. Inresponse to the input signals being applied thereto, the DUT 35 developsvoltage and current output signals. The output signals are then routedto a comparator 25 via the switching means 30, and any drop in thevoltage and/or current as initially applied to the DUT 35 is calculatedby the comparator 25. This drop in voltage and current is caused by theDUT 35, and is used to determine the measured resistance of the DUT 35or, more specifically, it is used to determine the measured resistanceof the network of the DUT 35 which exists between the particularterminals corresponding to the switching cells being accessed at thatgiven time.

The controller 40 provides the comparator 25 with user defined limitsfor comparing to the measured resistance. The comparator 25 then makes adetermination as to whether the network should be considered anopen-circuit or a short-circuit. The results are then fed to thecontroller 40 for further processing or outputting.

Preferably, for increased testing accuracy and speed, the source 15 andcomparator 25 are integrated as a single electronic component.

Referring now to FIG. 2, each switching cell 45,50 has a correspondingprobe 55,60 connected thereto for electrically interfacing with arespective terminal of a network 65 of the DUT 35. Each switching cell45,50 includes four field-effect transistors 70-105, which may be D-typeMOS transistors, as switching elements. Transistors 70,75,90 and 95 arereferred to herein as "from" transistors and transistors 80,85,100 and105 are referred to herein as "to" transistors. These transistors may beconfigured in the form of a 4-wire Kelvin-bridge type circuit. During atest, the current and voltage signals will flow from one of the cells,via the cell's "from" transistors, and be applied to the network 65 fordeveloping output signals. The output signals will then be received bythe other cell by way of the other cell's "to" transistors.

In operation, the controller 40 sends control signals to the gateterminals of the "from" transistors of one of the switching cells 45 or50, regulating these transistors to allow current to flow therethrough,i.e., the control signals will render the "from" transistors operationalor conductive for testing; and the controller 40 will also send controlsignals to the gate terminals of the "to" transistors of the otherswitching cell, rendering these transistors conductive. Thus, electricalsignals will be allowed to flow from the source to drain of the selected"from" and "to" transistors.

As a specific example, if the "from" transistors 70,75 of switching cell45 are selected by the controller 40, via line 130, to perform a test,and the "to" transistors 100,105 of switching cell 50 are selected bythe controller 40, via line 135, to perform the test, then current andvoltage input signals generated by the source 15 flow through lines 115and 110, and are applied to the network 65 via the "from" transistors70,75 of switching cell 45 and probe 55. In response to the inputsignals, the network 65 develops current and voltage output signals. Theoutput signals are output from the network 65 to probe 60, and receivedby the "to" transistors 100,105 of switching cell 50. These transistors100,105 route the output signals to the comparator 25 via lines 120 and125, so that the comparator 25 can determine if there is any drop incurrent and voltage. It is important to realize that this drop incurrent and voltage is not only caused by the resistance of the network65, but is also caused by the contact resistance existing at theinterfaces between the probes 55,60 and the terminals of the network 65.Furthermore, it should be noted that the electrical lines for routingthe voltage and current signals during testing also have resistance.However, this line resistance is known and constant and can thus bereadily accounted for during testing.

Although the switching cells 45,50 illustrated in FIG. 2 have theadvantages of high speed and low power requirements, the cells 45,50have the drawback of being limited in regard to the amount of testcurrent that can be applied to the network 65 via the transistors70-105. Specifically, the maximum allowable test current that can beused is approximately 20 mA for DC operation. In this regard, althoughincreasing test current may result in greater testing accuracy whenperforming continuity tests, increasing test current will also result inleakage of the transistors 70-105 during testing. Such leakage of thetransistors 70-105 causes inaccurate test results. Moreover, althoughincreasing test current would decrease contact resistance, the amount oftest current utilized during a test should not be increased because ofthis problem of leakage. Thus, the problem of contact resistance causinginaccurate test results, as outlined hereinabove, persists in thesetypes of test systems.

Reference is now made to FIG. 3 which illustrates the test system 10 ofFIG. 1 modified in accordance with the present invention. As shown,contact wetting means 140 is electrically connected between thevoltage/current source 15 and switching means 30, and also between thecomparator 25 and switching means 30. It is preferable, for purposes ofincreasing test speed and accuracy, that the source 15, the comparator25 and the contact wetting means 140 be integrated as a singleelectronic component.

The contact wetting means 140 comprises pulse means for generating apulse of increased magnitude, relative to the current and voltage inputsignals initially generated by the voltage/current source 15, forapplying to the DUT 35 and effectively electrically wetting theconnection or interface between the switching means 30 and the terminalsof the DUT 35 so that any resistance existing therebetween is decreasedor preferably eliminated.

In the preferred embodiment, the controller 40 is in electricalcommunication with the contact wetting means 140 so that the controller40 can regulate operation thereof. Thus, in order to conserve power andtest cycle time, the contact wetting means 140 can be designed to beinoperative during normal testing, and operative only when thecontroller 40 receives a signal indicating that a network of the DUT 35has a measured resistance which is higher than the limit set therefor.The controller 40 will then send a control signal so as to render thecontact wetting means 140 operative. The contact wetting means 140 willthen generate a pulse of increased magnitude, relative to the currentand voltage input signals generated by the source 15, and apply thepulse to the network of the DUT 35 so as to lower or eliminate anycontact resistance existing at the connection between the switchingmeans 30 and the terminals of the DUT 35. The network can then beretested to determine whether or not the initial high measuredresistance was due to contact resistance between the connection of theswitching means 30 and the terminals of the DUT 35. More particularly,if the network now has a measured resistance that is still higher thanthe limit, then there is reasonable likelihood that this high measuredresistance is caused by the resistance of the network itself; however,if the network now has a measured resistance that is lower than thelimit, then it can be appreciated that the original high measuredresistance was caused by the contact resistance, and not by theresistance of the network.

Those skilled in the electrical arts will appreciate that a multitude ofdifferent designs can be utilized for the contact wetting means 140. Forexample, the contact wetting means 140 can comprise amplificationcircuitry capable of developing a pulse using the signals generated bythe voltage/current source 15, or the contact wetting means 140 cancomprise a current source capable of independently generating therequired pulse. The contact wetting means 140 of a preferred embodimentas incorporated with the switching cells 45,50 of FIG. 2 is illustratedin FIG. 4 and described in detail hereinbelow.

Referring now to FIG. 4, the contact wetting means 140 comprises D-typeMOS field-effect transistors 145 and 150. The gate terminals oftransistors 145 and 150 are electrically connected to the controller 40via lines 155 and 160, the source and drain terminals of transistor 145connect to lines 110 and 115, and thereby connect to the voltage/currentsource 15; and the source and drain of transistor 150 connect to lines120 and 125, and thereby connect to the comparator 25. When thecontroller 40 sends a control signal rendering the transistors 145 and150 conductive, transistor 145 causes lines 110 and 115 to beshort-circuited, and transistor 150 causes lines 120 and 125 to beshort-circuited. Such short-circuiting of the lines 110 and 115 from thevoltage/current source 15 results in a paralleling of the "from" and"to" transistors 70-105 of each of the switching cells 45,50, and thenetwork 65. Thus, a single pulse of increased magnitude is developed bycombining the voltage and current signals generated by thevoltage/current source 15.

This single pulse is applied to the network 65 via one of the probes andvia the "from" transistors of one of the switching cells, and receivedby the other probe and the "to" transistors of the other switching cell.Thus, an electrical wetting of the interfaces between each probe 55,60and its corresponding terminal of the network 65 is accomplished, and alowering or elimination of contact resistance is realized. As mentionedabove, the network 65 can then be retested.

Typical test operation includes generation of approximately 20 mA ampsof current and approximately 100 volts of voltage by the source 15.Under these conditions, the normal test current applied to the network65 for testing is approximately 20 mA. However, the use of the presentinvention has provided for a contact wetting pulse of approximately 100mA, or about 5 times the normal test current. Application of such apulse to the interfaces between the probes 55,60 and the terminals ofthe network 65 has been shown to reduce contact resistance of between 1ohm and 35 ohms to less than approximately 1 ohm, thus providingincreased accuracy in testing by reducing false open-circuitdeterminations.

A further problem involves polarity sensitive contact resistance thatmay exist at the interfaces between the probes 55,60 and the terminalsof the network 65. Such a problem is caused by diode effects at theprobe interfaces. A solution to this problem is the application ofmultiple pulses of alternating polarity, i.e., AC pulses. Alternatingthe polarity of pulses can be accomplished by applying a pulse to thenetwork in one direction, then, subsequently, applying another pulse tothe network in the opposite direction. For example, referring to FIG. 4,the "from" transistors 70,75 of switching cell 45 and the "to"transistors 100,105 of switching cell 50 can be rendered conducting soas to allow a first pulse to be applied to the network 65 in onedirection. Then, subsequently, these transistors can be turned off andthe "from" transistors 90,95 of switching cell 50 and the "to"transistors 80,85 of switching cell 45 can be rendered conducting so asto allow a second pulse to be applied to the network 65 in the oppositedirection. Of course, as described hereinabove, each of the first andsecond pulses are developed by combining the voltage and current signalsgenerated by the voltage/current source 15.

FIG. 5 illustrates the method of operation in accordance with theinvention. The flowchart begins with a determination that the networkunder test has been measured to have a resistance which is above the setlimit. If a contact wetting pulse has already been applied to thecontacts or interfaces between the probes and terminals via thecurrently addressed "from" cell, then no further testing will beconducted and it is confirmed that the resistance of the network isabove the set limit. However, if a contact wetting pulse has not alreadybeen applied to the interfaces via the currently addressed "from" cell,then a pulse will be applied to the network for contact wetting, and thenetwork will be retested. If the retest determines that the resistanceof the network is not above the set limit, then the original test wasincorrect and the resistance of the network actually is not above theset limit, i.e., the original high measured resistance was caused bycontact resistance and not the network itself.

Further, if the retest determines that the measured resistance of thenetwork is above the set limit, then a further determination can be madeto determine whether or not the testing is being conducted in the ACcontact wetting pulse mode. If testing is not being conducted in the ACpulse mode, then no further testing is required and it is confirmed thatthe resistance of the network is above the set limit. However, iftesting is being conducted in the AC pulse mode, then the "from" and"to" switching cells will be reversed and a second contact wetting pulsewill be applied to the network, and the network will be retested again.More particularly, the original "from" cell will be used as the "to"cell for application of the second pulse, and the original "to" cellwill be used as the "from" cell for application of the second pulse.Thus, the second pulse can be applied to the network in the oppositedirection relative to the originally applied pulse and, subsequently,the network can again be retested.

While the invention has been described in terms of specific embodiments,it is evident in view of the foregoing description that numerousalternatives, modifications and variations will be apparent to thoseskilled in the art. Thus, the invention is intended to encompass allsuch alternatives, modifications and variations which fall within thescope and spirit of the invention and the appended claims.

What is claimed is:
 1. An improved test system of the type whichincludes a plurality of switching cells, each switching cell having aprobe which electrically interfaces with a terminal of a device undertest, and each switching cell being selectively and individuallyaddressable for selecting a first and second switching cell andcorresponding first and second terminal for allowing voltage and currentinput signals generated by a voltage/current source to be applied to anetwork of the device under test by way of the first switching cell andprobe and by way of the first terminal, the network responsivelydeveloping voltage and current output signals, and said second switchingcell and probe receiving said output signals by way of said secondterminal and routing said output signals to a comparator for comparingsaid output signals with said input signals for calculating differencesin voltage and current, for determining electrical resistance of thenetwork, wherein the improvement comprises:contact wetting means forgenerating a first pulse of increased magnitude, relative to the inputsignals, and applying said first pulse to the network by way of saidswitching cells and probes, and said contact wetting means furthergenerating a second pulse and applying said second pulse to the networkby way of said switching cells and probes in a direction of flow whichis opposite to the direction of flow of said first pulse, whereincontact resistance at the interfaces between said probes and saidterminals is effectively lowered.
 2. An improved test system accordingto claim 1, wherein said first pulse has a magnitude which is on theorder of 5 times greater than the magnitude of said input signals.
 3. Animproved test system according to claim 1, wherein said contact wettingmeans comprises pulse means for combining voltage and current inputsignals generated by the source, for generating said first pulse.
 4. Animproved test system according to claim 3, wherein said pulse meanscomprises a plurality of switches allowing for controllably combiningthe voltage signal with the current signal.
 5. An improved test systemaccording to claim 4, wherein said switches comprises transistors.
 6. Animproved test system according to claim 5, wherein said transistors areD-type MOS field effect transistors.
 7. An improved test systemaccording to claim 1, wherein said contact wetting means is operativewhen the network resistance is greater than a preset limit, andinoperative, when the network resistance is less than the preset limit.